The present invention relates to a display device for performing a display operation in accordance with signal light and, more particularly, to a display device designed to perform a high time-division drive display operation.
Display devices are classified into display elements which do not emit light by themselves, like liquid crystal display elements, but perform display operations by using external light such as light from backlights and the like, and self-luminescent or spontaneous display elements which perform display operations by emitting light by themselves. One example of the self-luminescent display elements is an organic electro-luminescent element (to be referred to as an organic EL element hereinafter) which has a luminescence mechanism having a luminescent layer consisting of an organic material and designed to emit light by using excitation energy generated by recombination of carriers, i.e., electrons and holes as particles having charges with different polarities, in the luminescent layer.
In a simple matrix type organic EL element having a plurality of scanning electrodes and signal electrodes formed on the upper and lower surfaces of the luminescent layer, which causes recombination, to extend in different directions, a voltage is sequentially applied to emit light. In this element, however, as the number of scanning electrodes is increased to realize a high-resolution display operation, the scanning time during which a scanning voltage is applied to one scanning electrode in 1 frame period is shortened, resulting in a decrease in emission luminance at each pixel. In addition, the luminescent layer emits light at the maximum luminance upon application of a voltage, but the luminance decreases instantaneously afterward. With this property, the luminescent layer has a poor memory function. For this reason, in some driving methods, the maximum luminance is increased to apparently maintain luminescence in 1 frame period. However, in order to obtain a sufficient contrast ratio at 1/100 duty, the maximum luminance must be set to about several thousand to 10,000 cd/m.sup.2. In this case, since crosstalk occurs, it is difficult to perform a proper display operation. Furthermore, since an excessive load is imposed on the luminescent layer, the luminescence life time is considerably shortened.
A technique of improving the memory function by using switching elements such as thin-film transistors (TFTS) and the like has been proposed. According to this technique, however, the switching elements lead to a decrease in the opening ratio (the ratio of the emission area), resulting in a decrease in emission luminance. The switching elements also complicate the manufacturing process, resulting in a decrease in yield. Furthermore, variations in the gate voltage/drain current characteristics of TFTs cause variations in display luminance among the respective pixels. Consider a substrate having TFTs. Since materials for elements constituting a TFT have high film formation temperatures, a thin, lightweight, flexible substrate such as a film substrate cannot be used. Such problems become more serious in elements having larger display areas.